Many commercially available gas sensors are of the amperometric type having two or more electrodes in which a catalytically active metal is fixed to a porous substrate. The porous substrate may operate as a gas permeable membrane and structural support for the electrode. The electrodes are located on the inside surface of the membrane where they make contact with an electrolyte such as sulfuric acid. External circuitry maintains one of the electrodes, the working electrode, at a selected electrical potential with respect to one of the other electrodes during operation.
When the gas of interest diffuses through the porous membrane to reach the working electrode, the diffused gas is oxidized or reduced at the interface of the working electrode and the electrolyte. That reaction generates an electrical current that is proportional to the concentration of the gas. In some cases, the gas of interest reacts with another chemical which, in turn, is oxidized or reduced at the electrode. In some cases, sensors are of a galvanic design wherein a metal such as lead is oxidized to provide the opposite current to that occurring at the working electrode.
In amperometric ethanol sensors in the prior art, a large platinum black electrode has been needed to form a stable signal, making the sensor bulky and difficult to miniaturize. In addition, the need for a large amount of noble metal for the electrode made the sensor expensive.
Also in the prior art, the sensors were connected to the external circuit through wires. For example, a platinum contact wire was connected to the catalytically active electrode and passed through the sensor body to an external contact. Since most sensors contain a corrosive, liquid electrolyte, a difficulty with sensors has been providing secure electrical contact with the electrodes while maintaining an electrolyte-tight seal at the location where the conductor passes through the sensor body. In the prior art, seals around conductors have included Teflon gaskets. In other methods, the seal has been made of thermoplastic material or epoxy resin.
U.S. Pat. No. 5,744,697 to Martell, et al. discloses a gas sensor of the type described above. The Martell, et al. gas sensor has a plastic housing comprised of a plurality of conductive housing portions integrally formed with a plurality of non-conductive housing portions. The housing has a receptacle disposed therein, and a gas-sensing agent is provided in the receptacle. A support sheet that has a plurality of electrodes formed thereon is disposed above the receptacle wherein electrical contact is made with electrodes to the conductive plastic portion, and a wick disposed in the receptacle causes the gas-sensing agent to maintain electrolytically conductive contact with the electrodes formed on the support sheet inside the sensor assembly.
The present invention uses electrodes that are in electrical contact with conductive plastic portions, as in the Martell, et al. patent, but in a design that uses simpler electrode and sensor housing construction. This results in a smaller sensor that is less expensive to manufacture, easier to interface to external circuitry, and less prone to leaking.